In evolved network architecture, the handover of an X2 interface takes place between Evolved NodeBs (eNodeBs). An eNodeB where a User Equipment (UE) is currently located is called a Source eNodeB (S-eNB). An eNodeB to which the UE is handed over is called a Target eNodeB (T-eNB). The handover is a process of handing over the UE from a cell controlled by the S-eNB to a cell controlled by the T-eNB.
The handover process in the prior art is as follows.
The UE context in the S-eNB includes roaming restriction information. The information is provided at the setup of a connection or the update of a last Tracking Area (TA).
According to the measurement result of the UE and the Radio Resource Management (RRM) of the S-eNB, the S-eNB determines to hand over the UE to a cell controlled by the T-eNB.
The S-eNB sends a handover request message to the T-eNB. The message carries mandatory information for the handover preparation on the T-eNB, including: MME UE S1AP ID (identity), old enb UE S1AP ID, target cell ID, Radio Resource Control (RRC) context, and System Architecture Evolution (SAE) bearer context. The T-eNB addresses the S-eNB and the Evolved Packet Core (EPC) according to MME UE S1AP ID or enb UE S1AP ID. The SAE bearer context includes the mandatory address information of the radio network layer and transport network layer, the Quality of Service (QoS) profile of the SAE bearer, and possible configuration information of the access layer. The T-eNB is configured with necessary resources.
The T-eNB implements admission control to improve the handover success possibility according to the received QoS profile of the SAE bearer. If the T-eNB is able to meet the resource requirement of the SAE bearer, the T-eNB allocates appropriate resources according to the received QoS profile of the SAE bearer and meanwhile, reserves a Cell-Radio Network Temporary Identifier (C-RNTI).
The T-eNB sends a handover request ACK message to the S-eNB. The message carries the newly allocated C-RNTI and parameters, for example, access parameters, Radio Network Layer (RNL) and/or Transport Network Layer (TNL) information for setting up a forwarding tunnel.
The UE receives a handover command message sent from the S-eNB. According to the message, the UE implements the handover process. The message carries the newly allocated C-RNTI and possible start time.
If the target cell is accessed successfully, the UE sends a handover confirm message to the T-eNB, indicating that the handover is complete. The T-eNB checks whether the C-RNTI in the message is allocated by itself.
The T-eNB sends a handover complete message to the EPC, indicating that the UE have changed the cell. The EPC hands over the data path to the T-eNB and releases the relevant user plane resources and transport network layer resources of the S-eNB.
The EPC sends a handover complete ACK message to the T-eNB to confirm that the handover is complete.
The T-eNB sends a release resource message to the S-eNB to trigger the S-eNB to release resources.
Upon reception of the release resource message, the S-eNB releases the radio resources and control plane resources related to the UE context.
In the above handover preparation process, if the radio link between the S-eNB and the UE fails, the UE changes the state during the subsequent handover. As a result, the handover is delayed and the system resources are wasted.